The present invention is directed to a device for automatically testing masks such as masks utilized in producing semiconductors which device utilizes a beam of coherent light and a detector arrangement.
The yield in the production of semiconductor components is often determined by the mask, which is used in exposing the light-sensitive layers, being free from faults. In the course of use, a mask frequently suffers damages such as indents, holes, scratches or becomes dirty due to adhering foreign bodies or particles. These faults or particles subsequently lead to the breakdown in the pattern being exposed on the component and render the component unusable for its intended purpose. In order to recognize in time that the mask has become unserviceable due to wear or from becoming dirty, and to be able to replace the mask, a mask testing arrangement which operates as rapidly as possible and as automatically as possible is desired.
Normally, masks are visually inspected under a microscope. However, on account of the extremely fine structure of the mask, a visual inspection is a time consuming operation. Thus, it was only possible to spot check portions of the mask and therefore only obtain statistical information concerning the state or condition of the mask.
It has been suggested to investigate the mask with the aid of an electro-optical deflected light beam which is focused with the aid of an objective lens onto the mask surface which is to be checked. The light which passes through the mask is intercepted by a photoelectric detector and is electrically analyzed. The light spot must execute a row by row scanning movement on the mask surface which scanning movement is achieved by the beam only partially illuminating the diameter of the objective lens and by the beam being conducted across the entire surface of the lens in a corresponding scanning motion.
This type of arrangement will only insufficiently fulfull the aims for testing the mask because of the following two reasons. In order to obtain the requisite scanning motion, the focusing lenses are only partially illuminated. Such illumination produces a scanning point which has a substantially poorer degree of resolution than the resolution which would be achieved by completely or fully illuminating the objective lens. Another difficulty with this proposed type of arrangement or device is that even a sophisticated objective lens does not permit the handling of an image field of the size of the entire semiconductor waffer with a resolution of the finest structures of the mask. Thus, this proposed device causes one to dispense with the discovery of the finer faults which may be present and on the other hand requires scanning times in the region or range of a number of minutes. Thus, only a relatively coarse testing of the mask can be carried out with this suggested device.